Point of taxation rule 5 for nov exam

Pls write ur querry with more clarification and do not wrie any short form and write very clearly

sir i want to say if a service is taxable for the 1st time in sept, 2012 1. Date of service is on 20 aug,12: date of invoice is on 2 sep,12 date of payment 30 aug,12-service is not taxable as per rule 5 of pot 2.Date of service 20 aug,12 date of invoice is on 10 sep,2012 date of payment 25 aug,12-service is not taxable as per rule 3 of pot. My qus is why in second case rule 3 apply.why rule 5 is not applicable in second case.